1. Field of the Invention
The present invention relates to a screen fabric and a method for making the same, which are employed in preparation of a precise stencil for screen printing of circuit-board and the like.
2. Description of the Prior Art
The screen fabric is stretched at a temperature of 100.degree. to 220.degree. C. to be stabilized in size and shape, which is termed "heat setting". In this heat setting process, the screen fabric is catched by its selvages or opposite end portions in warp direction by tenters of a tentering machine and stretched by the sames. In this heat setting process, the selvages of the screen fabric are held by means of pins or clips so that faults such as pinholes and collapses are produced in the screen fabric.
As shown in FIGS. 8 and 11, in conventional screen fabrics, selvages 3a, 3b have thicknesses of about 1.5 times larger than grounds 2a, 2b, which selvages 3a, 3b have normally widths of 10 to 15 mm and extend in warp direction of the screen fabrics 1a, 1b as indicated by an arrow "A" so as to be shaped into strip-like forms, opposite edges 4a, 4b of which selvages 3a, 3b have thicknesses of more than two times larger than the grounds 2a, 2b. As shown in a first conventional example in FIG. 8, the screen fabric is woven mainly of synthetic fibers of 7 to 50 denier. Consequently, although the selvages 3a, 3b are thicker than the grounds 2a, 2b, since the thickness of the selvage 3a is in a very thin order of 140 .mu.m, the screen fabric is apt to be torn in its portions where faults 3a' such as pinholes and collapses are present, when stretched.
In order to prevent the screen fabric from being torn in its stretching operation, as shown in a second conventional example in FIG. 11 in which the reference numeral 3b' denotes a pinhole, there has been proposed the selvage 3b provided with its stepped reinforcing portion called "selvage weave". However, such provision of the selvage weave is not sufficient in reinforcing the selvage 3b while disadvantageous in applying a sufficient tensile force for the screen fabric in the stretching operation of the same in warp direction as indicated by the arrow "A" because of the selvage weave being resistant to such stretching operation.
FIGS. 10 and 12 show results of the tensile strength test in warp direction of the first and second conventional examples, in which test the first conventional example shows 20.5 Kg on the average while the second conventional example shows 23.5 Kg on the average. In this test, specifications of specimens are as follows: THE FIRST CONVENTIONAL EXAMPLE
Mesh size (the number of fibers/inch): 250 to 252 warps/248 wefts PA0 Fiber diameters (.mu.m): warp.times.weft 40 PA0 Thickness of the fabric (.mu.m): 63 to 64 PA0 Width of the fabric (cm): 116.7 PA0 Fiber diameters (.mu.m): warp 40.times.weft 40 PA0 Thickness of the fabric (.mu.m): 63 to 64 PA0 Width of the fabric (cm): 115.4 PA0 1. to prevent the screen fabric from being torn in weft direction under the effect of a tensile force acting on the screen fabric in warp direction in the stretching operation of the fabric; PA0 2. to make it possible to apply a sufficiently large tensile force to the screen fabric in its stretching operation to provide a highly precise stencil for the screen printing of the circuit-board and the like; PA0 3. to apply a sufficiently large tensile force to the screen fabric so as to make it possible to provide a stencil having little elongation to increase the quality of the screen printing employing such stencil; and PA0 4. to prevent the wrinkles from occurring in the screen fabric when the fabric is wound up, to make it possible to obtain a stencil from the screen fabric and to improve the stencil in its appearance.